Interferometers are increasingly used in manufacturing environments for measuring surfaces, especially spherical or planar surfaces that can be represented by common wavefronts. A beam of coherent light is shaped to the desired wavefront and is divided into test and reference wavefronts. The test wavefront undergoes reflection from the test surface before being recombined with the reference wavefront. Any changes to the test wavefront are visible as an interference pattern produced by amplitude variations of the overlapping wavefronts.
The interference patterns, also referred to as interferograms, provide a map of phase differences between the two wavefronts. Periodic phase differences appear as bands of light, also referred to as "fringes". The spacing between fringes is a function of sensitivity of the interferometer. Cumulative changes in phase across the interference pattern are calculated using so-called "phase shifting" techniques that expose relationships between adjacent fringes. For example, the relative path lengths or the wavelengths of the test and reference wavefronts can be varied to determine if the phases of adjacent fringes are relatively advanced or retarded.
Sensitivity can be controlled by adjusting either the wavelength or the angle of incidence to maintain fringe density within a range that can be most effectively interpreted. The spacing between fringes of a single pass interferometer in which the test wavefront is reflected only once from the test surface corresponds to a height variation in the test surface equal to the wavelength divided by twice the cosine of the angle of incidence. At normal incidence, the height variation represented by an adjacent fringe is equal to one-half the wavelength. At higher angles of incidence, the adjacent fringes represent greater height variations and are spaced farther apart.
Higher angles of incidence are also used to measure surfaces having low specular reflectivity. For example, surfaces much rougher than optical quality can be measured at grazing incidence, which is defined herein as a non-normal angle of incidence sufficient to produce specular reflection. The higher angle of incidence also reduces sensitivity for maintaining fringe density within acceptable limits.
Commonly assigned U.S. Pat. No. 4,325,637 to Moore discloses a single-pass interferometer for measuring flat surfaces at grazing incidence. A light source in combination with a collimator produces a planar wavefront that is transmitted by a prism located adjacent to a test surface. One portion of the planar wavefront refracts from a prism reference surface as a test wavefront that strikes the test surface at grazing incidence, and another portion of the planar wavefront reflects from the same reference surface as a reference wavefront. The reflected test wavefront recombines with the reference wavefront on return through the reference surface. An interference pattern produced by the recombined wavefronts is projected onto a detector for analysis of test surface flatness.
Recessed surfaces that are very shallow can be similarly measured, but the side walls of deeper recessed surfaces block both incoming and outgoing rays--a phenomenon referred to as "shadowing". For example, recessed surfaces having aspect ratios of length to height less than 30 to 1 cannot be measured at all using incident angles of 86 degrees or more, which are common for measuring non-specular surfaces. Significant portions of shallower surfaces are also obscured. Such shadowing can be reduced by reducing the angle of incidence, but this option is often precluded by surface roughness or low specular reflectivity.